A. Field of the Invention
The present invention relates generally to threaded connections for tubular goods, and more particularly to a threaded connection for use on tubular goods of bimetallic construction as used in the production of oil and gas.
B. Description of the Prior Art
It has become increasingly necessary to produce oil and gas from reservoirs containing significant amounts of H.sub.2 S and/or CO.sub.2. Serious corrosion and embrittlement problems can occur when conventional carbon steel tubular goods are used in such environments. Corrosion inhibitors have been used, but these are costly and not always effective. The industry has considered using tubulars made of entirely of stainless steel, nickel alloys, or even titanium alloys. However, such materials, while generally having high performance characteristics, are very expensive and may require special handling techniques.
The most satisfactory solution developed so far involves the use of bimetallic tubes, which include a high strength outer pipe or shell with an inner liner or pipe of a corrosion resistant material. Such tubes offer the advantages of having load-bearing and rugged-handling characteristics of conventional steel tubes with the corrosion resistance of high-alloy materials.
Since oil field tubular goods come in discrete lengths, it is necessary to connect them together to form strings. The connections must be likewise protected against corrosion or embrittlement. There have been manufactured for use with bimetallic pipe couplings of solid corrosion resistant material, such as stainless steel. However, such stainless steel couplings lack the strength and rugged handling characteristics of ordinary couplings. There have been developed for use with bimetallic pipe threaded connections of ordinary steel but with means for protecting the connection material. Examples of such connections are disclosed in U.S. Pat. No. 3,307,860; U.S. Pat. No. 3,336,054; U.S. Pat. No. 4,026,583; and U.S. Pat. No. 4,373,750.
In addition to being able to withstand the corrosive effects of sour gas environments, pipes and their connections must be able to withstand service in deep high-pressure wells. Wells in excess of 15,000 feet in depth and 20,000 pounds per square inch in pressure are not uncommon. Accordingly, pipes and connections must have high tensile strength and the connections must provide effective leak proof seals. Additionally, oil field tubular goods are often subjected to rough treatment when they are connected together on site. The threaded connections must be resistant to galling, jamming, cross threading, and mechanical damage to sealing surfaces.
It is therefore an object of the present invention to connect together bimetallic pipe in such way that the internal structure presents a continuous metalurgical barrier while the external coupling maintains the mechanical properties of the pipe. It is a further object of the present invention to provide a threaded connection for use in sour gas environments that has a tensile strength approaching that of the pipes being interconnected. It is a further object of the present invention to provide a threaded connection for use in sour gas environments that is substantially leak proof at all operating pressure. It is a further object of the present invention to provide a threaded connection that is resistant to galling, jamming, cross threading, and mechanical damage.